1. Field of the Invention
The present invention relates to an image taking apparatus that performs image taking for generating an image signal by capturing, with an image pickup device, object light sent via an image taking optical system.
2. Description of the Related Art
A charge coupled device (CCD) is widely used as an image pickup device for a digital camera, one of image taking apparatuses. The CCD is provided with plural photoelectric conversion elements, such as a photodiode, for converting incident object light into an image signal, which is an electrical signal. Furthermore, many CCDs are provided with a lot of microlenses arranged corresponding to plural photoelectric conversion elements in order to increase the amount of incident light into each of the lot of photoelectric conversion elements. In such CCDs, the rate of condensation of light into a photoelectric conversion element differs depending on the angle of the incident light into a microlense. For example, the larger the angle of the incident light into the microlense is, the farther the portion where light is condensed is away from the center of the photoelectric conversion element. Furthermore, there may be caused a phenomenon that, when the object light passes through an image taking optical system constituted by an image taking lens and the like, the object light that passes through the periphery of the image taking optical system is darkened in comparison with the object light that passes through the vicinity of the central axis of the image taking optical system. Therefore, when uniform object light, for example all in red, is received by a CCD via an image taking optical system, difference is caused among image signals from respective photoelectric conversion elements even though the object light was uniform. As a result, there is caused a shading phenomenon on an image on the display screen which has been obtained through image taking, that color differs depending on positions on the screen.
There is also arranged a color filter for acquiring color signals of red (R), green (G) and blue (B) on the CCD. In this case, since infrared rays in object light passes through the color filter, there is generally arranged an infrared-ray cutting filter (an IR filter) for filtering out the infrared rays at the front of the CCD. However, the shading phenomenon may be caused after all on an image on the display screen which has been obtained through image taking, because of cut-off frequency difference or transmittance difference at a particular frequency caused due to manufacture difference of the infrared ray cutting filter.
The angle of incident light into microlenses constituting the CCD changes depending on the diaphragm opening of a camera lens. Accordingly, there is proposed a technique as follows. That is, sensitivity values of a CCD corresponding to diaphragm values of a camera lens are stored as correction data, and when an image is taken, gain of an amplifier for amplifying an image signal outputted from the CCD is adjusted with the correction data to correct the sensitivity of the CCD, and thereby shading is corrected (see Japanese Patent Laid-Open No. 06-178198).
In the technique proposed in the Japanese Patent Laid-Open No. 06-178198, shading correction is performed by adjusting gain of an amplifier with data for sensitivity correction depending on the diaphragm value of a camera lens for all the pixels and respective regions on the screen where an image is displayed. Therefore, there is a problem that a long processing time (operation time) is required for performing the shading correction, and therefore a long image taking time is required. Furthermore, shading correction is performed for all the pixels and respective regions on the screen, and therefore there is also a problem that the S/N ratio, the ratio of necessary strength of an image signal to strength of unnecessary noise, is deteriorated. In addition, shading correction by adjusting gain of an amplifier for respective regions on the screen leads to uneven S/N ratio and thus makes it difficult to obtain a high-quality image.